public class TestForm extends AbstractForm {

  private Integer data;

  public TestForm() throws ProcessingException {
    super();
  }

  public void startModify() throws ProcessingException {
    startInternal(new ModifyHandler());
  }

  public MainBox getMainBox() {
    return getFieldByClass(MainBox.class);
  }

  public NummerField getNummerField() {
    return getFieldByClass(NummerField.class);
  }

  @Order(10.0)
  public class MainBox extends AbstractGroupBox {

    @Order(10.0)
    public class NummerField extends AbstractIntegerField {

      @Override
      protected Integer execValidateValue(Integer rawValue) throws ProcessingException {
        MessageBox.showYesNoCancelMessage("theres", "nothingto", "seehere");
        return rawValue;
      }
    }

    @Order(20.0)
    public class CancelButton extends AbstractCancelButton {
    }
  }

  public class ModifyHandler extends AbstractFormHandler {
    @Override
    protected void execLoad() throws ProcessingException {
      importData();
    }
  }

  public void setData(Integer data) {
    this.data = data;
  }

  private void importData() {
    getNummerField().setValue(data);
  }
}

    @Order(20.0)
    public class BearbeitenMenu extends AbstractMenu {

      ...

      @Override
      protected void execAction() throws ProcessingException {
        TestForm form = new TestForm();
        form.setData(2);
        form.startModify();
        form.waitFor();
      }
    }

@Order(10.0)
public class IntegerField extends AbstractIntegerField {

  private Integer m_localValue;
  /**
   * The guard is used to ensure the message box is not shown in an infinite loop
   */
  private AtomicBoolean m_guard = new AtomicBoolean(false);

  /**
   * This method is currently needed since the getValue is blocked during a validation process.
   */
  @Override
  protected void valueChangedInternal() {
    super.valueChangedInternal();
    m_localValue = getValue();
  }

  @Override
  protected Integer execValidateValue(final Integer rawValue) throws ProcessingException {
    if (!m_guard.get()) {
      new ClientSyncJob("", ClientSyncJob.getCurrentSession()) {
        @Override
        protected void runVoid(IProgressMonitor monitor) throws Throwable {
          if (MessageBox.showYesNoCancelMessage("theres", "nothingto", "seehere") == IMessageBox.YES_OPTION) {
            try {
              m_guard.set(true);
              setValue(rawValue);
            }
            finally {
              m_guard.set(false);
            }
          }
        }
      }.schedule();

      // ensure the old value is returned
      return m_localValue;
    }
    return rawValue;
  }
}